ABSTRACT
Formation damage has been a constant headache to the oil producing industries as it is
considered an impairment of the permeability of petroleum bearing formation with an
expensive remediation procedure. Although, the prevention of formation damage is
impracticable since every single operation embarked upon in petroleum production is a
potential source of damage, it could be controlled. In this project, a well was studied and BHP
survey was used from BHP analysis in addition to the information of the well history and
reservoir data available. The well was observed to have been damaged with a skin of 115 and
a damage ratio indicating the well should have been flowing about two times its present
production rate. There are two major stimulation procedures which are the hydraulic fracturing
and the matrix acidization in which the latter was used in the case of the damaged well. Well
57XX had a production rate which was initially 1550bbl/day at its peak before undergoing a
decline, increased to 2100bbl/day and then continued to flow at an average of 2000bbl/day
before a sharp decline and subsequent gradual declination of production rate showing the effect
formation damage had on the well 57XX. This in conclusion proved that the matrix acidization
technique used to stimulate the well was effective as it led to an increase in the well
permeability and hence, increased the oil production rate.
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TABLE OF CONTENTS
CERTIFICATION…………………………………………………………………………..i
DEDICATION……………………………………………………………………………….ii
ACKNOWLEDGEMENT………………………………………………………………….iii
ABSTRACT…………………………………………………………………………………iv
TABLE OF CONTENTS……………………………………………………………………v
LIST OF TABLES…………………………………………………………………………vii
LIST OF FIGURES………………………………………………………………………viii
NOMENCLATURE………………………………………………………………………..ix
CHAPTER ONE: INTRODUCTION
1.1 General Background………………………………………………………………………1
1.2 Statement of Problem…..…………………………………………………………………4
1.3 Purpose of the Study………………………………………………………………………4
1.4 Relevance and Justification……………………………………………………………….5
1.5 Scope of Project…………………………………………………………………………..6
CHAPTER TWO: LITERATURE REVIEW
2.1 Formation Damage Mechanisms…………………………………………………………7
2.1.1 Fluid-rock incompatibility……………………………………………………7
2.1.2 Fluid-fluid incompatibility…………………………………………………..11
2.2 Stimulation Method……………………………………………………………………..14
2.2.1 Matrix Acidization…………………………………………………………..14
2.2.2 Hydraulic Fracturing…………………………………………………………15
2.3 Important Parameters for Evaluation of Formation Damage……………………………15
2.3.1 Permeability…………………………………………………………………16
2.3.2 Productivity Index (J)……………………………………………………….16
2.3.3 Skin Factor (S)………………………………………………………………16
2.3.4 Flow Efficiency (FE)………………………………………………………..16
2.3.5 Damage Ratio (DR)…………………………………………………………17
2.3.6 Transmissibility……………………………………………………………..17
2.3.7 Radius of Investigation………………………………………………………17
2.4 Methods for Recognition of Formation Damage………………………………………..17
2.4.1 Production history review……………………………………………………18
2.4.2 Pressure transient well test analysis…………………………………………18
2.4.3 Comparison of production performance of offset wells…………………….18
2.4.4 Drill stem tests………………………………………………………………19
2.4.5 Electric logs…………………………………………………………………19
2.4.6 Production records and logging……………………………………………..20
2.4.7 Laboratory core tests analysis………………………………………………20
CHAPTER THREE: METHODOLOGY
3.1 Matrix Acidization………………………………………………………………………24
3.2 General Well History and Status of Well RayXX………………………………………24
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3.3 Matrix Acidization Stimulation Procedure………………………………………………25
3.4 Required Data……………………………………………………………………………26
3.5 Background Information On Well and Reservoir………………………………………27
3.5.1 Well Data……………………………………………………………………27
3.5.2 Reservoir Data………………………………………………………………28
3.5.3 Data before Matrix Acidization……………………………………………..29
3.5.4 Data after Matrix Acidization………………………………………………30
CHAPTER FOUR: RESULT AND DISCUSSION
4.1 Analysis of Result………………………………………………………………………..33
4.2 Discussion of Result…………………………………………………………………….34
4.2.1 Production……………………………………………………………………34
4.2.2 Total Skin……………………………………………………………………34
4.2.3 Pressure Drop Due to Skin………………………………………………….34
4.2.4 Permeability…………………………………………………………………35
4.2.5 Productivity Index…………………………………………………………..35
4.2.6 Flow Efficiency……………………………………………………………..36
4.2.7 Damage Ratio……………………………………………………………….36
4.2.8 R-Factor……………………………………………………………………..37
4.2.9 Transmissibility……………………………………………………………..37
4.3 Economic Evaluation……………………………………………………………………37
CHAPTER FIVE: CONCLUSION AND RECOMMENDATION
5.1 Conclusion………………………………………………………………………………39
5.2 Recommendation………………………………………………………………………..40
REFERENCES…………………………………………………………………………….42
APPENDIX…………………………………………………………………………………46
vii
LIST OF TABLES
Table 3.1: A table showing the available well data for well RAY XX………………………..27
Table 3.2: A table showing the available reservoir data………………………………………28
Table 3.3: A table showing the production rate and other parameters before stimulation…….29
Table 3.4: A table showing the new production rate and other parameters after stimulation…30
Table 3.5: A table showing the pre-acid and post-acid parameters……………………………31
Table 4.1: Table showing the estimated parameters of the well gotten from the MATLAB
program for before stimulation and after stimulation…………………………………………33
viii
LIST OF FIGURES
Figure 2.1: Formation damage classification…………………………………………………13
Figure 2.2: Schematic diagram of stimulated well by matrix acidization. (Pandey, 2011)……14
Figure 3.1: Graph of production rate and GOR against date………………………………….29
Figure 3.2: Graph of production rate and GOR against date………………………………….30
Figure 3.3: Well pressure analysis as determined by Hagedorn and Brown correlation……31
Figure 4.1. Graph showing production and permeability…………………………………….35
ix
NOMENCLATURE
S = Skin factor
Sd = Skin due to damage
H = Total reservoir thickness
µ = Viscosity
q = Oil production rate (bbl/day)
ra = Effective well bore radius (ft)
re = Damage radius (ft)
rw = Well bore radius (ft)
ΔP = Change in pressure (psi)
ΔPskin = Pressure drop due to damage
skin
Pwf = Flowing well pressure (psi)
Pr = Reservoir pressure (psi)
P = Initial pressure (psi)
K = Permeability (md)
Kh = Horizontal permeability
(md)
Kv = Vertical permeability (md)
Ka = Average permeability (md)
BS&W = Basic sediment and water
(%)
GOR = Gas oil ratio (scf/bbl)
Bhp = Bottom hole pressure (psi)
Bopd = Barrel of oil per day
Bo = Oil formation volume factor
(rb/stb)
Bg = Gas formation volume factor
API = America Standard Institute
Ct = Total compressibility factor
(psi-1
)
PI = Productivity Index
FE = Flow efficiency
DR = Damage ratio
POOH = Pulling out of hole
TVD = True vertical depth
MD = Measured depth
BOP = Blow out preventer
BHA = Bottom hole assembly
M/U = Make up
R/U = Rig up
RIH = Run in hole
OD = Outer diameter
ID = Internal diameter
1
CHAPTER ONE
1.0 INTRODUCTION
1.1. General Background
Formation damage is generally considered as the impairment of the unseen by the
inevitable, causing an unknown reduction in the unquantifiable (Petrowiki, 2015). Also, it is a
condition which occurs when barriers to flow develop in the near-wellbore region to give rise
to a lower than expected production rate from or injection rate into a hydrocarbon bearing
reservoir rock and it requires interdisciplinary knowledge and expertise (Amaefule et al, 1988).
It can also be referred to as an impairment to reservoir (reduced production) permeability
caused by wellbore fluids used during drilling, completion and work over operations
(Petrowiki, 2015).
Oil well productivity on the other hand, is generally considered as the ability of a
reservoir to produce hydrocarbons after the well has been drilled and made ready for
production. The production stage of oil is the most important stage of a well’s life because it
determines if the aim of drilling such well has been achieved or not, and this can be measured
by the quantity of crude oil derived or quantity of crude oil which is producible. Formation
damage is one of the major causes of decrease in oil production as a result of damage to the
formation by reducing its porosity and permeability which also leads to flow restrictions. Flow
restrictions into the wellbore create additional pressure drops known as ‘skin’ and reduce well
productivity.
Formation damage is known to occur during any stage of a well’s life; from initial
exploration, through appraisal, through production and through secondary or tertiary recovery
and all these have their various roles which they play in the reduction of oil well productivity.
Formation damage indicators include, among others, permeability impairment, skin damage
2
and decrease of well performance. Formation damage according to Porter (1989) is considered
not necessarily reversible and what gets into the porous media does not necessarily come out.
It is, therefore, better to avoid the occurrence of formation damage rather than trying to restore
it. Models for formation damages which have been proven to be verified can be used to avoid
or minimize it (Faruk, 2011). Carefully planned laboratory and field tests can also help in
providing scientific guidance as well as develop strategies for minimizing the damage. It will,
therefore, cause considerable cost for remediation and deferred production. Accurately
designed experimental and analytical techniques with the modelling and simulation approaches
can be used to understand the evaluation, prevention, remediation and the control of formation
damage that leads to low oil productivity.
Formation damage can occur as a result of fluid/rock incompatibility; particle migration
and deposition may occur as a function of the chemistry of the clay minerals and the chemical
and electrochemical nature of both the natural formation fluid and the drilling fluid. Changes
in the pore fluid can also induce clay swelling which in turn reduces the pore spaces in the
reservoir and this is considered a form of damage to the formation as it reduces the productivity
of the formation.
The occurrence of the fluid/rock incompatibility is not as a result of only swelling of
the clay and particle migration and deposition. Formation damage can also occur as a result of
the fluid/fluid incompatibility. The incompatibility of the introduced fluid (drilling fluid) and
the reservoir pore-fluid which creates emulsion blocks can only be controlled by stimulation
techniques that include pre-flush or after flush techniques. Formation damage caused by
various fluids introduced into the well is remediated by careful treatment design and quality
control. The departure from radial flow in a homogenous and isotropic medium can also be a
cause of formation damage. A positive skin may arise from a reduction of the area available to
flow and/or a departure from purely radial flow (Harper and Buller, 1986).
3
Formation damage also has other causes such as the mechanical deformation around a
borehole or perforation tunnel, reduction of fluid pressure during production, etc. Thorough
understanding of the formation damage mechanism’s stringent measures for its control and
prevention, and effective and efficient treatments are the keys for optimum production
strategies for oil and gas fields.
The consequences of formation damage are the reduction of the oil and gas productivity
of reservoirs and noneconomic operation. Hence, once formation damage has occurred, it is
necessary that proper assessment, planning and treatment will require the cooperative efforts
and knowledge of the geologists, reservoir engineer and production engineer both in the field
and in the laboratory. This combined effort and approach will therefore help to develop
effective solutions to the damage. A wide knowledge of the mechanism of formation damage
is necessary in order for the engineers and geologists to develop effective, preventive and
mitigating procedures.
With recent improvements in technology, laboratory, geology and engineering, it is
easier to achieve accurate measurements which can provide the necessary insights into the
mechanism, prevention and effective treatment of formation damage (Amaefule et al., 1988).
Confidence in formation damage prediction using models cannot be achieved without
undergoing field testing as they are necessary for the verification of the models. After the
verification of the model, it can then be applied for accurate simulation of the reservoir
formation damage and designing effective measures for formation damage (Faruk, 2011).
Formation has varying characteristics and a formation damage model can be used to
incorporate these variations into a history matching process for the characterization of reservoir
systems which can also be used for accurate prediction of future performance. Recent literature
surveys have had various arguments and debate about if formation damage is considered more
4
detrimental for the vertical wells or for the horizontal ones. However, the fact still remains that
in both cases, the production loss due to formation damage is significant.
1.2. Statement of Problem
Formation damage over the years has proven to be a great concern in the minds of oil
industries as it is considered inevitable and is capital intensive if incurred. These concerns have
become more prevalent as we come in contact with many more challenging reservoirs utilizing
even more challenging drilling, completion and production methods. After drilling and
completion of a well, it is essential for the reservoir engineer to make accurate and essential
estimation of the productivity index of the well which is a function of several parameters, some
of which include pressure, flow-rate, etc. One of the most important parameters which shows
how far a well has been damaged or stimulated is the skin. However, the derivation of these
parameters has been difficult and erroneous due to formation damage which might have
occurred during drilling, completion, work-over etc. It is therefore, necessary to study and
determine ways in which formation damage can be reduced since studies have shown it cannot
be totally eradicated.
1.3. Purpose of the Study
The aim of the project is to study the effect of formation damage on oil well productivity
with a case study of the Niger Delta Region. Specifically, this study will:
i. Take a look at the major causes of formation damage with a case study of the Niger
Delta Region.
ii. Study the effects of formation damage on well productivity, and
iii. Propose possible solutions to various types of damages which can be encountered
in the oil fields of the Niger Delta Region.
5
1.4. Relevance and Justification
Formation damage has been one of the prominent problems in the oil and gas industry
as poor understanding of the reservoir has affected the productivity of the reservoir and has
caused several problems ranging from early water production, reduction in oil production as a
result of damaged or reduced permeability, etc. which could ultimately lead to the destruction
of the reservoir or total abandonment of the well. This has also caused several companies much
more expenses than budgeted in order to come up with a good remedy for the reservoir and to
determine other means of increasing productivity. So it is of paramount importance that the
petroleum industry is enlightened about the negative effects of formation damage and ways it
can easily be detected since it can be corrected if quickly diagnosed. Some causes also can be
prevented while some cannot; that is why it is of great relevance and also very important to
discuss the causes of formation damage and ways of rectifying it considering both time, cost
and safety. As a result of this, this project will highlight the impacts of formation damage on
oil well productivity, assess these impacts, provide remedial solutions, taking time and cost
into consideration.
Furthermore, this project includes several practical importance, relevance and uses,
some of which are:
i. It is of great importance as it helps the oil and gas companies to optimize production
from the well.
ii. It also helps to increase the life of the well by being able to identify the possible
damages and provide means of preventing it.
iii. It helps to reduce cost, i.e., the amount spent by companies to provide remedial
solutions to damaged wells and expenses used to ensure the recovery of more
hydrocarbons from already damaged wells.
6
iv. It also helps to ensure accurate time constrains by helping to prevent work over,
stimulation operations or total abandonment of the well.
v. It provides highlight on how drilling, completion, perforation and other treatment
methods can cause skin and affect the well.
1.5. Scope of Project
Formation damage in the petroleum industry has been a major problem contributing to
the issues concerning the production of hydrocarbons from the reservoir. Therefore, it is very
important for its effect to be analyzed in order to pinpoint the available ways of correction.
This project is limited to the study of data on damaged wells under production from an
oil and gas producing company in the Niger Delta Region. The required data will consist of the
particular type of damage which occurred to the well, the means of detection of the damage,
the time constraint in which the damage was identified and rectified, the remedial solution to
the particular type of damage which in this case is matrix acidization.
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